Low density mist collector pad

ABSTRACT

A wire mesh mist collector pad having multiple density zones, the zones being arranged other than to provide a constantly increasing density gradient, provides a reduced pressure drop and increased capacity. For example, the pad can include at least three zones with the zones arranged so that the density gradient in the direction of gas flow through the pad varies from low to high to low or from high to low to high.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/728,909 filed Mar. 22, 2010, and co-owned at the time of filing thisapplication that issued as U.S. Pat. No. 8,083,827 on Dec. 27, 2011, andthat was a continuation of then co-owned U.S. application Ser. No.11/446,656 filed Jun. 5, 2006, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improved mesh for removing entrained orsuspended droplets or particulates in a gas stream.

2. The State of the Art

Many chemical and other industrial processes create a gas stream thatincludes solid and/or liquid particles that must be removed from the gasstream. These particles are typically liquid droplets created byentrainment, impingement, chemical reaction, or condensation, but canalso be solid particles.

Perhaps the most typical of methods for removing these unwantedparticles is a device which causes the gas/particle flow to changedirection. Because the particle has a higher density than the gas of thestream, the momentum of the particle will tend to make the particletravel in a straight line and not change direction as quickly as thegas. For example, in the case of a stream of water droplets in air, thegas stream can be passed through a wire mesh: the liquid droplets cannotnegotiate the tortuous path through the mesh and so they land on andadhere to the mesh, and coalesce, by surface tension, and then run offfrom the mesh due to gravity. Gas-liquid separations that use demistingpads include gas plants, refineries, steam-generating power plants, gasscrubbers, and various other operations used in producing petrochemicalsand speciality chemicals.

Another device typically used for removing liquid or solid particlesfrom a gas stream includes a series of wave plates (vanes) arrangedparallely, each vane being a thin sheet that is formed into hills andvalleys. The vanes are arranged spaced closely together. The gas streamenters one side and takes a zig-zag path to reach the other side. Theentrained droplets cannot negotiate the rapid zig-zag and impinge on thevane, where they cling and run down the wall. While vanes are morerobust, they are significantly heavier and more costly to manufacturethan a wire mesh mist collector.

The typical industry standard for mesh pads is to use 0.011 inch wire ata density of 9 lb./ft³. The wire commonly used is 304SS, or any suitablealloy, metal, or plastic chemically compatible with the process stream.Some operations use a multilayered pad having the lowest density padupstream and the highest density pad downstream. Although termed“multilayered,” the pad actually has multiple zones, where each zone iscomprised of one or more layers of the same mesh. Multilayer padspresently used in the industry have two or three zones at most. The padmust remove entrained material but without causing any significantpressure drop. In a typical plant, a pressure drop of one inch of wateracross a pad amounts to hundreds of thousands of dollars a year in a onebillion SCF/d plant to push the gas around with the increased pressuredrop.

SUMMARY OF THE INVENTION

In light of the foregoing, among various objects of this invention areto make a pad that has good if not improved capacity, to make the padlighter and so use less material, thereby making the pad less costly tomanufacture, and to provide a lower pressure drop across the pad,thereby reducing the operating cost of the processing facility, andespecially providing all of these benefits in a single device.

In summary, this invention provides a multizone mesh pad having a lowerdensity and a higher capacity than existing pads, and which is made witha smaller diameter wire. The pad is made by knitting the wire into atube, flattening the tube in one direction, then crimping the tube in adifferent direction, optionally crimping the tube in yet a differentorientation, forming cut lengths of various crimped mesh tubes into apad by stacking the lengths in a desired order to having alternatinghigh and low density zones, and fixing the pad to a grid forinstallation in process equipment.

In a particular embodiment, this invention provides a multilayer padhaving at least three zones of alternating high and low density mesh,each zone comprising at least one layer of mesh, more preferably atleast four zones, even more preferably at least five zones, and mostpreferably at least six zones. The mist collector pads of this inventiondo not have an increasing density gradient across the entire pad, butinstead alternate the density gradient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-10 depict results of testing the multilayer pad of thisinvention against other separation devices, with the comparativepressure drop data shown in odd-numbered figures and comparativeentrainment data shown in even-numbered figures.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The mesh used to make the pads of this invention is made with aconventional knitting machine that knits wire instead of thread.Although it is preferred in this invention to use a thinner wire, suchas 0.006″ as compared with the 0.011″ wire conventionally used in theindustry, the advantages obtained by alternating high and low densitymeshes to make a multilayer pad provides improved capacity and a lowerpressure drop. The wire must be chosen to be inert in the environment ofuse, and type 304SS or 304L is typically used when droplets are removedfrom an aqueous or organic stream; 316L, Alloy 20, andpolytetrafluoroethylene-based polymers (TEFLON) are used for sulfuricacid; 410SS and glass are used for mild chemicals; a nickel-copper alloy(MONEL) is used for corrosive chemicals, nickel is used for caustic; andthe like as is known in the industry.

Multilayer pads in the prior art have a density gradient increasing inthe direction of flow; for example, a lower density pad upstream and ahigher density pad downstream. Surprising it has been found that simplyvarying the density between adjacent layers of a multilayer pad providesimproved capacity and a lower pressure drop. Thus, alternating from highto low to high, or low to high to low, provides improvements. As shownbelow in Example 4, in comparison with a pad having the same totalthickness using meshes of lower density (5 lb/ft³ and 9 lb/ft³) than theembodiment of the invention in Example 4 (9 lb/ft³ and 12 lb/ft³ meshdensities), the present invention provides a lower pressure drop and hasa higher capacity.

The pads of this invention are first made by knitting a wire, such as0.006″ type 304SS wire, on a conventional knitting machine to produce aknitted tube or sock. The knitted tube is first flattened by being runbetween roller. The flattened pad is then crimped by being run betweentwo rollers, at least one having a pattern (like an embossing pattern)thereon so that the mesh tube is flattened and crimped. The crimped meshtube may optionally then be crimped in a different direction ororientation between a second pair of rollers, having the same or adifferent pattern, the rollers oriented differently with respect to themesh than the first pair. For example, if the first pair of crimpingrollers is disposed horizontally, the mesh is fed horizontally; then,without changing the orientation of the mesh, it is run between a secondpair of crimping rollers disposed vertically. The two pairs of crimpingrollers need not be orthogonal to each other, although that ispreferred; it is enough that the orientation of the second pair isdifferent than the orientation of the first pair with respect to a giventube orientation. The resulting tube has a lower density than theoriginal knitted and uncrimped tube. By varying the number of wires usedto knit the tube and the number of layers of mesh used to make a givenlayer in the mist collector, as well as the number of crimpingoperations, a layer having a desired density can be produced. Increasedcrimping lowers the final density of the mesh. In the examplesfollowing, one mesh style (3BF) is made from 0.006″ wire into a meshlayer having a density of about 7.2 lb/ft³ (120 ft²/ft³; 98.6% voids),and another mesh style (3BA) is made with the same wire diameter into amesh layer having a density of 12.0 lb/ft³ (200 ft²/ft³; 97.6% voids).

The mesh layers are then layered or stacked in a desired order on aframe or grid that both supports the multilayer pad and connects it tothe process vessel (and holds it in place against the gas flow). Thegrid is secured and the mist collector is then installed into theprocess equipment.

EXAMPLES

A sample multilayer pad according to this invention was compared with aconventional multilayer pad on a testing apparatus. In the testingapparatus, a mixture of air and water was used as the test stream, usinga 15 HP radial blade blower with an inlet damper for air flow control, a12 inch., Sch. 20, 16 foot long exit pipe from the blower, and using aDwyer model DS-400-12 multi-orifice flow sensor for air flowmeasurement, all for supplying a vertical test chamber. The outlet fromthe test chamber had a 40 inch long Sch. 20 pipe including a FilterSenseModel LM-70 Mist Gauge for entrainment measurement. The pressure dropwas measured with an inclined manometer (measurement in inches of watercolumn), the test system temperature having been measured with a WekslerInstruments dial thermometer with 1° F. gradations. The inlet loadingwas applied with either full cone water (Bete SCM9SQ), but morepreferably, as in these examples, using a two-fluid (Spraying Systems1/2J+SU 79) air-water spray nozzle. The water was recycled to the inletusing a 2HP Teel centrifugal pump and metered using a zero to five GPMrotameter. The multilayer pad devices were tested to determine theircapacity to eliminate water drops from the air stream. Capacity wasdetermined by the air velocity at which breakthrough started to occur.The amount of water in the exit stream was determined using an electricinduction probe (model LM 30, ProFlow brand series, from ImpolitEnvironmental Control Corp., Beverly, Mass.). The outputs of the probewere used to calculate entrainment ENTR as gal_(water)/mmSCF_(air)(gallons of water per million standard cubic feet of air).

The sample pad (designated below as “LDP”) was constructed by layeringthe aforementioned style 3BA and 3BF meshes as follows, all being madewith 0.006″ 304SS wire, to make the pad designated as “LDP” herein, allof the 3BF layers having been crimped twice and the 3BA layers havingbeen crimped once. A “layer” in the pad construction is a single crimpedknit tube. The sample LDP had the following construction (the directionof gas flow being bottom to top):

TOP   6 layers of 3BF   1 layer of 3BA   2 layers of 3BF   1 layer of3BA   2 layers of 3BF   1 layer of 3BA   3 layers of 3BF   1 layer of3BA   6 layers of 3BF BOTTOMThe direction of the gas flow in the testing apparatus is verticallyfrom bottom to top. The total thickness of the multilayer pad was sixinches and the average density was 2.9 lb/ft³.

Example 1

The LDP mist collector pad was tested against a six inch thick style 4CApad, made with 0.011″ wire, having a density of 9 lb/ft³, 85 ft²/ft³surface area, and 98.2% voids, layered to provide a thickness of sixinches. As shown in FIG. 1, the LDP pad has significantly less pressuredrop over the entire air velocity range, and FIG. 2 shows thatbreakthrough on the 4CA pad occurred at about 19 ft is air velocitywhereas the LDP pad experienced breakthrough at about 23 ft/s.

Example 2

An eight inch comparison device was made (upstream to downstream) usingfive inches of a conventional vane separator (VH12), one inch of 7CAmesh, and two inches of 3BF style mesh. The style 7CA mesh is made from0.011″ wire, has a density of 5.0 lb/ft³, a surface of 45 ft²/ft³, and99.0% voids. This VH127CA3BF pad was tested against the LDP pad of thisinvention. As shown in FIG. 3, the LDP pad had a lower pressure dropthroughout the air velocity range, and FIG. 4 shows the pads havecomparable breakthrough, yet the LDP pad is only three-quarters thethickness of the comparison pad.

Example 3

A six inch pad (2″7CA4″4CA) was made with two inches of 7CA style meshand four inches of style 4CA mesh to produce a six inch pad. This wastested against the six inch LDP pad. As shown in FIG. 5, the LDP pad hada lower pressure drop through the flow range, and FIG. 6 shows thatbreakthrough occurred at a higher air velocity in the LDP pad than withthe comparison pad.

Example 4

The present six inch LDP pad was tested against eight inches of standardundulating vanes with a spacing of one-half inch between vanes. As shownin FIG. 7, the pressure drop was slightly better for the vanes than theLDP pad, although FIG. 8 shows that breakthrough occurred at a higherair velocity using the LDP pad than the vanes.

Example 5

The present LDP pad was tested against an eight inch pad made using(upstream to downstream) one inch of the 7CA style mesh, two inches of3BF style mesh, and five inches of the conventional vane (similar toExample 3 but reordered). FIG. 9 shows that the novel LDP pad exhibiteda lower pressure drop over the entire air flow range and FIG. 10 showsthat the LDP pad experiences breakthrough at about the same air velocityas the comparison pad.

These examples show that a multilayer pad having more layers than in theart provides a lower pressure drop, higher capacity, or both, incomparison with a separation device having a lower density and/orgreater thickness. Only a thicker set of conventional vanes provided alower pressure drop along much of the velocity range, yet breakthroughoccurred at a lower velocity for the vanes than for the novel LDP pad.

The foregoing description is meant to be illustrative and not limiting.Various changes, modifications, and additions may become apparent to theskilled artisan upon a perusal of this specification, and such are meantto be within the scope and spirit of the invention as defined by theclaims.

What is claimed is:
 1. A knitted wire mesh mist collector pad having anaverage density (measured in terms of mass per volume of the knittedwire mesh collector pad) and comprising at least three zones offlattened and crimped wire mesh tube, at least two of said zones havingdensities (measured in terms of mass per volume of the knitted wire meshin each of said at least two zones) that are different from one anotheras a result of comprising meshes of different densities (measured interms of mass per volume of the knitted wire mesh) and the at leastthree zones arranged so that a density gradient in the direction of gasflow through the pad varies from low density to high density to lowdensity and comprising: (a) a first zone of the mist collector padincluding one or more stacked layers of a knitted wire mesh tube thathas been (i) flattened and (ii) crimped twice; (b) a second zone of themist collector pad including one or more stacked layers of a knittedwire mesh tube that has been (i) flattened and (ii) crimped once; (c) athird zone of the mist collector pad including one or more stackedlayers of a knitted wire mesh tube that has been (i) flattened and (ii)crimped twice; (d) the first, second, and third zones secured in a gridwhich is used to install the mist collector pad in a process gas streamso that during use of the mist collector pad, the first zone is upstreamof the second zone and the second zone is upstream of the third zone;and wherein: (I) the density of the second zone (measured in terms ofmass per volume of the second zone) is greater than the density of thefirst zone; (II) the density of the second zone is greater than thedensity of the third zone; and (III) the wire meshes of the first,second, and third zones are selected so that during use of the mistcollector pad, liquid droplets in a process gas stream land on the wiremeshes, coalesce on the meshes, and then run off from the meshes due togravity.
 2. The mist collector pad of claim 1 where the average densityof the mist collector pad (measured in terms of mass per volume of themist collector pad) is 2.9 lb/ft³.
 3. A process vessel comprising themist collector of claim
 1. 4. A knitted wire mesh mist collector padcomprising: (a) a first zone of the knitted wire mesh mist collector padhaving one or more layers of a knitted wire mesh tube that has been (i)flattened and (ii) crimped twice; (b) a second zone of the knitted wiremesh mist collector pad having one or more layers of a knitted wire meshtube that has been (i) flattened and (ii) crimped once and wherein: thedensity of the second zone (measured in weight per unit volume of thesecond zone) is greater than the density of the first zone (measured inmass per unit volume of the first zone); (c) a third zone of the knittedwire mesh mist collector pad having one or more layers of a knitted wiremesh tube that has been (i) flattened and (ii) crimped twice andwherein: the density (measured in weight per unit volume) of the secondzone is greater than the density (measured in mass per unit volume) ofthe third zone; and (d) the first, second, and third zones secured in agrid which is used to install the knitted wire mesh mist collector padin a process gas stream so that during use of the knitted wire mesh mistcollector pad, the first zone is upstream of the second zone and thesecond zone is upstream of the third zone; and (e) wherein the wiremeshes of the first, second, and third zones are selected so that duringuse of the mist collector pad, liquid droplets in a process gas streamland on the wire meshes, coalesce on the meshes, and then run off fromthe meshes due to gravity.
 5. The knitted wire mesh mist collector padof claim 4 wherein each of the one or more layers of the first zone isconstructed of the same type of knitted wire mesh tube that has been (i)flattened and (ii) crimped twice as each of the one or more layers ofthe third zone of knitted wire mesh tube that has been (i) flattened and(ii) crimped twice.
 6. The knitted wire mesh mist collector pad of claim4 wherein the first, second, and third zones are contiguous.
 7. Theknitted wire mesh mist collector pad of claim 4 comprising: (a) a fourthzone of the knitted wire mesh mist collector pad having one or morelayers of a knitted wire mesh tube that has been (i) flattened and (ii)crimped once and wherein: the density of the fourth zone (measured inweight per unit volume) is greater than the density of the first zoneand the density of the fourth zone is greater than the density of thethird zone; (b) the fourth zone is secured in the grid so that duringuse, the third zone is upstream of the fourth zone; and (c) wherein thewire mesh of the fourth zone is selected so that during use of theknitted wire mesh mist collector pad, liquid droplets in a process gasstream land on the wire mesh, coalesce on the mesh, and then run offfrom the mesh due to gravity.
 8. The knitted wire mesh mist collectorpad of claim 7 wherein each layer of the first zone is the same as eachlayer of the third zone.
 9. The knitted wire mesh mist collector pad ofclaim 7 wherein each layer of the second zone is the same as each layerof the fourth zone.
 10. The knitted wire mesh mist collector pad ofclaim 9 wherein each layer of the first zone is the same as each layerof the third zone.
 11. The knitted wire mesh mist collector pad of claim7 wherein the first, second, third, and fourth zones are contiguous. 12.A knitted wire mesh mist collector pad comprising: (a) a first zone ofthe mist collector pad having one or more layers of a knitted wire meshtube that has been (i) flattened and (ii) crimped once; (b) a secondzone of the mist collector pad having one or more layers of a knittedwire mesh tube that has been (i) flattened and (ii) crimped twice,wherein the density of the second zone is less than the density of thefirst zone; (c) a third zone of the mist collector pad having one ormore layers of a knitted wire mesh tube that has been (i) flattened and(ii) crimped once, wherein the density of the second zone is less thanthe density of the third zone; (d) the first, second, and third zonesheld in a grid which is used to install the mist collector pad in aprocess gas stream so that during use of the mist collector pad, thefirst zone is upstream of the second zone and the second zone isupstream of the third zone; and (e) wherein, the wire meshes of thefirst, second, and third zones are selected so that during use of themist collector pad, liquid droplets in a process gas stream land on thewire meshes, coalesce on the meshes, and then run off from the meshesdue to gravity.
 13. The knitted wire mesh mist collector pad of claim 12wherein each layer of the first zone is the same as each layer of thethird zone.
 14. The knitted wire mesh mist collector pad of claim 12wherein the first, second, and third zones are contiguous.
 15. Theknitted wire mesh mist collector pad of claim 12 comprising: (a) afourth zone of the mist collector pad having one or more layers of aknitted wire mesh tube that has been (i) flattened and (ii) crimpedtwice; and (b) the fourth zone secured in the grid so that during use,the third zone is upstream of the fourth zone; and wherein: (I) thedensity of the fourth zone is less than the density of the first zone;(II) the density of the fourth zone is less than the density of thethird zone; and (III) the wire mesh of the fourth zone is selected sothat during use of the mist collector pad, liquid droplets in a processgas stream land on the wire mesh, coalesce on the mesh, and then run offfrom the mesh due to gravity.
 16. The knitted wire mesh mist collectorpad of claim 15 wherein each layer of the first zone is the same as eachlayer of the third zone.
 17. The knitted wire mesh mist collector pad ofclaim 15 wherein each layer of the second zone is the same as each layerof the fourth zone.
 18. The knitted wire mesh mist collector pad ofclaim 17 wherein each layer of the first zone is the same as each layerof the third zone.
 19. The knitted wire mesh mist collector pad of claim17 wherein the first, second, third, and fourth zones are contiguous.